The present invention relates to a probe head for a coordinate measuring machine and to a coordinate measuring machine using such a probe head. More particularly, the invention relates to a probe head comprising two axes of rotation for positioning a probing element, such as a stylus, in a desired rotary position about one or two axes of rotation.
A coordinate measuring machine in terms of the present invention is a device for recording selected measurement points on a measurement object using the probe head and for determining coordinates of any selected measurement point relative to a predefined coordinate system. The coordinates represent the position of the respective measurement point in the defined coordinate system. The coordinate system is often a three-dimensional coordinate system, such that the measurement point coordinates represent the spatial position of the measurement point. Determination of a large number of measurement point coordinates for a large number of measurement points makes it possible to measure geometric properties of a workpiece, for example the diameter of a bore, the distance between two workpiece edges, or the height of a projection on the workpiece. Even a complex 3D free-form of a workpiece, such as the form of a turbine blade, can be measured by recording a large number of measurement points on the workpiece.
Typically, the probe head of the coordinate measuring machine is movable within a defined measurement volume relative to the workpiece holder. With the aid of the probe head, the selected measurement points on the workpiece are detected. The measurement point coordinates are then determined on the basis of the position of the probe head relative to the workpiece holder and on the basis of the position of the probe head relative to the detected measurement points. Therefore, the probe head generally has a sensor, with the aid of which the position of the probe head relative to one or more selected measurement points can be determined. The sensor often comprises a probe element, in particular a stylus with a spherical end, with the aid of which a selected measurement point is touched. In addition, however, there are also probe heads with contactless sensors, for example with optical sensors, which determine the position of a measurement point relative to the probe head using methods of image processing, interferometry and/or laser triangulation.
EP 0 317 967 A2 discloses a coordinate measuring machine with a double-axis rotary joint, with the aid of which the sensor can be brought to different orientations (angled settings) relative to a workpiece. This makes it possible to bring a stylus to an optimal position relative to a workpiece feature, for example in order to measure a horizontally extending bore on the workpiece. As an alternative to the tactile probe head, a contactless triangulation probe head could be secured on the rotary joint and could likewise be brought to a favorable orientation relative to a workpiece.
The coordinate measuring machine with the rotary joint according to EP 0 317 967 A2 has proven useful in many practical applications. Nonetheless, there are measurement tasks for which the known coordinate measuring machine is still not optimal, for example because an optimal orientation of the stylus with the aid of the rotary joints during a continuous probing process is not easily possible.
U.S. Pat. No. 7,503,125 B2 discloses a coordinate measuring machine, with a probe head that is coupled to the coordinate measuring machine via a six-axis rotary joint. Such a rotary joint permits almost any desired spatial orientations of the probe head. However, such a rotary joint is very complex and heavy and takes up quite a lot of space within the available measurement volume of a coordinate measuring machine.
US 2006/0010701 A1 discloses a probe head for a coordinate measuring machine, wherein the probe head itself has two integrated rotary drives so as to be able to bring the stylus into different orientations. In some exemplary embodiments, the stylus is laterally offset with respect to one or both rotation axes in order to permit different height positions in the vertical direction when a stylus is pivoted into the horizontal. However, the addition of a further rotation axis or the replacement of the probe element by another sensor are not readily possible in this probe head. Consequently, this probe head too is not optimal in terms of its possibilities of use.
A further probe head with an integrated rotary unit is known from EP 1 602 900 A2. The known probe head has two electrical drives, which are not only needed to change the current orientation of the stylus, but also to hold the stylus in its current orientation. Accordingly, the electrical drives have to be supplied with current during a measurement with the probe head, and this permanently generates heat in the probe head. The replacement of the sensor or the addition of a further rotation or pivot axis also appear to be problematic in this case.
DE 10 2007 022 326 A1 discloses a coordinate measuring machine with a passive rotary joint, i.e. with a rotary pivot joint that does not require integrated electrical drives. The rotary joint has two toothed wheels which are accessible from the outside and which can each be turned by means of the probe head being driven linearly with respect to a rack with the aid of the drives of the coordinate measuring machine frame. This passive rotary mechanism thus minimizes heat input in the area of the probe element, as is desirable with regard to measurement precision. However, adjustment of the probe element is time-consuming here and can only be conceivably done with difficulty during a probe procedure.
Generally, there is a desire that a sensor for detecting measurement points on a measurement object can be spatially oriented as flexibly as possible in order to be able to detect the greatest possible number of different measurement points on a workpiece without having to change the position of the workpiece. It is also desirable to be able to use different probe heads with different sensor principles and/or probe elements on one coordinate measuring machine, since the different sensor principles and probe elements each have different advantages and disadvantages, and therefore different probe elements and sensor principles may be advantageous for a specific measurement purpose. Moreover, it is desirable if the probe head of a coordinate measuring machine is as light and as compact as possible, so that it permits rapid movements of the coordinate measuring machine in the measurement process and takes up very little of the available measurement volume.